17.1Operating System ConceptsCSMC 412CSMC 412Operating SystemsProf. Ashok K Agrawala© 2006 Ashok AgrawalaSet 77.2Operating System ConceptsDeadlocksDeadlocks The Deadlock Problem System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock27.3Operating System ConceptsObjectivesObjectives To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks To present a number of different methods for preventing or avoiding deadlocks in a computer system.7.4Operating System ConceptsThe Deadlock ProblemThe Deadlock Problem A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set. Example z System has 2 tape drives.z P1and P2each hold one tape drive and each needs another one. Example z semaphores A and B, initialized to 1P0P1wait (A); wait(B)wait (B); wait(A)37.5Operating System ConceptsBridge Crossing ExampleBridge Crossing Example Traffic only in one direction. Each section of a bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). Several cars may have to be backed up if a deadlock occurs. Starvation is possible.7.6Operating System ConceptsSystem ModelSystem Model Resource types R1, R2, . . ., RmCPU cycles, memory space, I/O devices Each resource type Rihas Wiinstances. Each process utilizes a resource as follows:z request z use z release47.7Operating System ConceptsDeadlock CharacterizationDeadlock Characterization Mutual exclusion: only one process at a time can use a resource. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task. Circular wait: there exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1is waiting for a resource that is held by P2, …, Pn–1is waiting for a resource that is held by Pn, and P0is waiting for a resource that is held by P0.Deadlock can arise if four conditions hold simultaneously.7.8Operating System ConceptsTraffic DeadlockTraffic Deadlock57.9Operating System ConceptsResourceResource--Allocation GraphAllocation Graph V is partitioned into two types:z P = {P1, P2, …, Pn}, the set consisting of all the processes in the system.z R = {R1, R2, …, Rm}, the set consisting of all resource types in the system. request edge – directed edge P1 → Rj assignment edge – directed edge Rj→ PiA set of vertices V and a set of edges E.7.10Operating System ConceptsResourceResource--Allocation Graph (Cont.)Allocation Graph (Cont.) Process Resource Type with 4 instances Pirequests instance of Rj Piis holding an instance of RjPiPiRjRj67.11Operating System ConceptsExample of a Resource Allocation GraphExample of a Resource Allocation Graph7.12Operating System ConceptsResource Allocation Graph With A DeadlockResource Allocation Graph With A Deadlock77.13Operating System ConceptsResource Allocation Graph With A Cycle But No DeadlockResource Allocation Graph With A Cycle But No Deadlock7.14Operating System ConceptsBasic FactsBasic Facts If graph contains no cycles ⇒ no deadlock. If graph contains a cycle ⇒z if only one instance per resource type, then deadlock.z if several instances per resource type, possibility of deadlock.87.15Operating System ConceptsMethods for Handling DeadlocksMethods for Handling Deadlocks Ensure that the system will never enter a deadlock state. Allow the system to enter a deadlock state and then recover. Ignore the problem and pretend that deadlocks never occur in thesystem; used by most operating systems, including UNIX.7.16Operating System ConceptsDeadlock PreventionDeadlock Prevention Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources. Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources.z Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none.z Low resource utilization; starvation possible.Restrain the ways request can be made.97.17Operating System ConceptsDeadlock Prevention (Cont.)Deadlock Prevention (Cont.) No Preemption –z If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released.z Preempted resources are added to the list of resources for which the process is waiting.z Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.7.18Operating System ConceptsDeadlock AvoidanceDeadlock Avoidance Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need. The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition. Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes.Requires that the system has some additional a priori information available.107.19Operating System ConceptsSafe StateSafe State When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state. System is in safe state if there exists a safe sequence of all processes. Sequence <P1, P2, …, Pn> is safe if for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j<I.z If Piresource needs are not immediately available, then Pican wait until all Pjhave finished.z When Pjis finished, Pican obtain needed resources, execute, return allocated resources, and terminate. z When Piterminates, Pi+1can obtain its needed resources, and so on. 7.20Operating System ConceptsBasic FactsBasic Facts If a system is in safe state ⇒ no
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